Addiction is a chronic relapsing disorder. Despite extended abstinence, addicts may experience intense craving in response to drug re-exposure, cues or stress. How do strong cravings re-emerge and what are the neurobiological triggers? Nucleus accumbens (NAc) is a key target of addictive drugs in the mammalian brain. Animal models implicate NAc in enduring vulnerability to reinstatement of drug seeking. Although reinstatement involves plasticity in NAc AMPA-type glutamate receptors (AMPARs), the identity of this plasticity is unclear. Combining rodent reinstatement models with NAc whole-cell recordings in an ex vivo preparation, we identified a putative neural substrate for relapse. During cocaine abstinence, a cocaine prime, in vivo or in vitro, induces AMPAR long-term depression (re-exposure LTD), indicating that NAc AMPAR plasticity in response to environmental stimuli during abstinence is highly dynamic. We hypothesize that re- exposure LTD provides a synaptic gateway for reinstatement. To test this, we will directly measure and manipulate NAc AMPAR plasticity in drug-, cue- and stress-primed reinstatement and incubation models. In addition, priming in a dish gives us a tractable model system to study molecular mechanisms of reinstatement-linked plasticity. We hypothesize that propping up NAc AMPAR function during abstinence may be a useful tool in combating relapse. The aim of this K02 proposal is to provide an opportunity for career development and training in state-of-the-art research methods to support our program studying the neurobiology of addiction relapse. Specifically, I plan to develop expertise in two areas: 1) the use of optogenetics as a means to probe pathway-specific plasticity in neural reward circuits and 2) gold standard preclinical addiction relapse models in mice. This expertise will directly enhance my current R01-funded research described above. Furthermore, this opportunity to stay abreast of new approaches for modeling human diseases in experimentally tractable species and for measuring and manipulating neural circuit plasticity is critical for long-term success in my research field.